This recipe will please not only the meat-and-potato lover in your house but will also please the cook. It comes together in just 30 minutes with minimal cleanup! We seared tender, juicy strip steaks in a hot cast-iron skillet, then made a fast pan sauce with mustard, wine and herbs. Mashed potatoes are kicked up with sour cream, horseradish and spinach for a stick-to-your-ribs side dish that's a cinch to prepare in the microwave.
Put the potatoes and 2 tablespoons water in a large microwave-safe bowl. Cover tightly with plastic wrap and poke a few holes in the top. Microwave until the potatoes are tender and can be easily broken apart with a fork, about 15 minutes.
Carefully remove the plastic (watch out for hot steam) and add the sour cream, horseradish, 4 tablespoons butter, 1 teaspoon salt and some black pepper to the bowl and mash until creamy. Fold in the spinach until wilted and well combined.
Rub both sides of each steak with the garlic clove, then put the clove in a large cast-iron skillet. Season the steaks on both sides with 2 teaspoons salt and a generous amount of black pepper.
Add the olive oil to the skillet and heat over medium-high until shimmering. Add the steaks and cook, undisturbed, until deep golden-brown and no longer sticking to the pan, about 5 minutes. Flip the steaks and continue cooking until golden-brown and an instant-read thermometer registers 125 degrees F when inserted into the center for medium-rare. Remove the steaks to a cutting board to rest for 10 minutes before slicing. Discard the garlic clove.
Meanwhile, turn off the heat and let the pan cool for about 2 minutes. Turn the heat to medium-low and add the shallots. Cook, stirring occasionally, until softened and golden-brown, about 2 minutes. Stir in the Dijon and thyme leaves until well combined. Pour in the white wine and simmer until thickened and reduced by half, 4 to 5 minutes. Swirl in the remaining 2 tablespoons butter until melted and silky.
Thinly slice the steaks crosswise and divide among plates. Spoon sauce over the steak and serve with the potatoes.